Landfills are seminatural terrestrial ecosystems reconstructed on lands degraded by waste disposal. They are unique in terms of site formation, nature of stratum, and biological activities, but vary according to their age, waste composition, engineering design, and ecological practice. From an environmental perspective, landfills are depositories for municipal solid wastes (sanitary landfills) and less frequently hazardous wastes (secure landfills). Landfills are ubiquitous, as sanitary landfilling is the most common method of municipal solid waste management worldwide. Landfill leachate is formed when rainwater infiltrates and percolates through the degrading waste, while landfill gas is a microbial degradation byproduct under anaerobic conditions. Modern landfills are designed and engineered to restrict the formation and movement of landfill leachate and gas, and to minimize environmental nuisance caused by wind-blown litter, pests, and odor during operation. These landfills, either the containment or entombment type, have buried waste that is isolated from the environment. Older landfills are of the dilution and attenuation type that makes use of the substratum for pollution mitigation; they are unconfined with no facilities for leachate treatment and gas extraction. With dilution and attenuation landfills, problems associated with leachate and gas are common. In terms of environmental biotechnology, landfills can be regarded as large-scale bioreactors in which the organic matter in the buried waste is anaerobically degraded to produce landfill gas which is methane-rich and can be used for electricity generation.
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